Modern wireless communications networks are commonly arranged in a “cellular” configuration, wherein a base station (also known as eNode B, or eNB) provides wireless communication services to subscribers, for example using mobile terminals (also known as User Equipment, or UE) within a geographic area, called a cell or sector. Most modern systems utilize coherent processing, wherein a receiver is assumed to be able to estimate the radio channel from a transmitter, and to take advantage of the channel quality information to more accurately detect transmitted data from received signals. To enable the receiver to estimate the channel, a transmitter that transmits a series of known (to the receiver) data patterns, known as reference signals (RS) or demodulation reference signals (DMRS) (referred to herein as simply RS). Using a priori knowledge of the RS data pattern, the receiver can estimate distortion effects injected by the wireless communication channel, then use these channel estimates to better extract data from wireless communication signals received from the same transmitter.
Particularly in the uplink (UL) direction (i.e., from UE to eNB), due to the large number of UE in a cell, the RS transmitted by different UE towards the same eNB are subject to interference from each other. Additionally, UL transmissions in a cell are subject to interference from transmissions in neighboring cells. Many modern wireless communication networks support a hierarchical arrangement of smaller cells (e.g., micro-cells or pico-cells) within the geographic area of a cell (known as heterogeneous networks, or HetNet). In these environments, UL RS transmissions from different UE are subject to even greater interference.
To mitigate or eliminate UL RS interference, the data patterns comprising the RS may be made orthogonal to each other. For example, a base sequence common to a plurality of UE—such as by being derived from a cell-ID—may be subjected by each UE to a different cyclic time shift (CS) value. As another example, orthogonal cover codes (OCC) multiplex RS based on orthogonal time domain codes. Although these techniques are effective in multiplexing RS assigned to UE transmitting on fully overlapping bandwidths, orthogonality is lost when the bandwidths differ and/or when an interfering UE employs another base sequence. With OCC, the problem is that it is limited to low mobility and only two UEs.
Modern wireless communication technology developments give rise to further problems regarding the reception and use of RS from UE. Multiple Input Multiple Output (MIMO) techniques utilize multiple antennas on transmitters and/or receivers to improve reception accuracy and/or increase data rates through spatial multiplexing of multiple data streams. Coordinated Multipoint Processing (CoMP) is a technique wherein the scheduling and/or signal processing of transmissions in multiple cells are coordinated to improve link quality. These technological developments require scheduling flexibility and improved channel estimation quality, even for geographically far away UE belonging to another cell. Making cell coverage more dense (e.g., HetNet), increasing the number of receive antennas, and CoMP processing emphasize the need for orthogonal RS between different cells, without affecting scheduling flexibility and multiplexing capacity of the RS.